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Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664
Posted: 09:30am 02 Jun 2018
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Hi All,
Just considering dropping my outdoor fridge & freezer of the power after dark.
800 litres combined; fridge freezer & upright freezer & even though they might not chew much overnight in the sub 5° to -5° stuff we get here, I'm still curious about what their defrost cycles do.
Have checked one fridge & it's rated at 120W. Defrost rating in 575W.
But when does it do that? And more so when will it do it if I stuck a timer on them both & cut their power between say 10PM & 9AM.
Or is it more a case of don't bother....
Cheers
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 09:46am 02 Jun 2018
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My fridge has a thermistor sensor attached to the evaporator, but outside the forced air circulation path through the evaporator fins. I suspect a very small heating current passes through that thermistor so that as long as the sensor (and evaporator) are ice free it stays out of defrost.
If the evaporator turns into a solid block of ice and blocks the airflow, then this magic sensor will be pulled down in temperature to the evaporator and ice temperature, and trigger a defrost cycle.
Its a matter of thermal conductivity. As long as the sensor is in still air but close to the evaporator, even very cold air it will stay slightly warmer due to self heating. If the sensor becomes encased in solid ice, it cools down much more.
At least that is how I think it works, there seems to be no other mechanism, or other possible use for that sensor.
I doubt if cutting the power at night is going to seriously upset things.Edited by Warpspeed 2018-06-03Cheers, Tony.
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 10:35am 02 Jun 2018
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If it is an old school fridge the defrost timer is a motor driven timer. Defrost occurs about once every 12 hours when the timer does a full cycle. Turning off the fridge will just pause it and it will continue where it left off. If the fridge is all electronic it will probably wait the defrost interval from when the power comes on. The thermistor on the evaporator is there to maintain +5 degrees while defrosting, there is also a thermal fuse in all defrost circuits to prevent a fire in case the defrost gets stuck on. The defrost does not wait for solid ice, in normal operation, there should be no more than a thin layer of ice crystals.
Letting the temperature rise in freezer above minus 15 degrees is not recommended as it reduces the time the food remains safe even though it is still below 0.Edited by Madness 2018-06-03There are only 10 types of people in the world: those who understand binary, and those who don't.
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166
Posted: 12:29pm 02 Jun 2018
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I have a "fridge only" refrigerator and that has no defrost heater it sheds the water between every compressor run. Very frugal on power, it would be even better with an external condenser.
Freezers are all over the shop, my big chest freezer (provided there is enough cold mass inside) will hold its temp for a couple of days, the only mod is a foam mattress chucked on the lid. I have an upright with no drawers that was so pathetic I now use it to store welding consumables in the shed.Edited by yahoo2 2018-06-03 I'm confused, no wait... maybe I'm not...
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 09:26pm 02 Jun 2018
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My Fridge/freezer is my biggest consumer of power 2.5 - 3 KWH per day. There are much more efficient ones available, I saw a review of a Panasonic side by side with ice maker that uses 1 KWH per day. Can't buy it in Australia they don't import them as they said they would not sell enough. QLD government was offer rebates for energy efficient appliances, I went looking at new fridges and no side by sides met the star rating required to get the rebate.
Most fridges/freezers now have the condenser coil under the outer skin of the fridge so you can't insulate them better like Yahoos foam mattress on the top of the freezer. It works there as the lid does not have the coil under the skin. Since I have discovered I can play around with refrigeration in the future I may take the fridge we have and add a separate condenser and add a 100mm layer of insulation to the fridge. Or I could just get more PV panels and a bigger battery bank. There are only 10 types of people in the world: those who understand binary, and those who don't.
Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664
Posted: 11:20pm 02 Jun 2018
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Both probably fall into the electronic control era. The 2 door upright, 420L is about 10 years old & the 360L freezer about 5.
Freezer was sitting at -25°C before my temp display failed, so figure there won't be a lot of overnight drop in winter weather.
Did take not once of what it lost in warmer weather when we had an 8 hour planned outage, but don't remember what I saw.
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 11:42pm 02 Jun 2018
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I can constantly log power consumption through my "smart meter". At night there is never very much running except for the refrigerator, and the defrost element consumes three times as much power as the compressor (350W and 120W). So the defrost cycle really stands out as a power spike.
The defrost cycle is very erratic, so there is far more going on than just a software timer that defrosts at regular fixed intervals. When it does do a defrost cycle the volume of melt water is always fairly constant, which makes me think it can somehow "know" when ice accumulation has reached a certain stage.
As the whole thing is controlled by some mystery software, its not really possible to know for sure exactly how it decides when to defrost.Cheers, Tony.
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 12:02am 03 Jun 2018
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Some fridges monitor door open time as that is a big contributor to the amount of frost. You can't make frost from dry air in the fridge, it needs to come from somewhere else.There are only 10 types of people in the world: those who understand binary, and those who don't.
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166
Posted: 01:25am 03 Jun 2018
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there is a drawback with extra insulation, it is possible to reach the dewpoint on the metal skin under the new insulation then moisture can build up and rust the surface. the foam on the lid is damp underneath in some weather.
I guess it could be sealed with a spray on plastic dip product before the insulation is applied. or just let it rust! I'm confused, no wait... maybe I'm not...
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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